1. Field of the Invention
The present invention relates to a polyarylene sulfide, a process for preparing the polyarylene sulfide and a resin composition containing the polyarylene sulfide.
The polyarylene sulfide is high in non-Newtonian behavior and high-degree of branching, not too high in an average molecular weight despite the high degree of branching, and a broad molecular weight distribution. The polyarylene sulfide further exhibits excellent moldability, flowability and thermal stability, is so colorless that it has a high degree of whiteness, and very small molding flash (burr) in injection molding. Furthermore, it is readily prepared as a stable resin in its characteristic properties by controlling its polymerization system. The polyarylene sulfide thus prepared can provide a resin composition with improved properties including mechanical strength, thermal resistance, long-term stability and chemical resistance.
2. Description of Related Art
Polyarylene sulfides such as polyphenylene sulfide re resins which are thermoplastic yet partially thermosetting, and they are generally excellent in resistance to heat and chemicals and in dimensional stability so that they lend themselves to molding materials as engineering plastics, for example, for electric and electronic parts, injection-molded articles for precision parts, films, and so on.
It is known that polyarylene sulfides prepared by conventional processes are of a thermally cross-linked type and of a linear high-molecular-weight type.
The polyarylene sulfides of the thermally cross-linked type present various disadvantages. A dark color is imparted to the polyarylene sulfide as well as ultra-high-molecular-weight component results as a polymer powder is heated in the presence of oxygen in order to render a molecular weight of the polyarylene sulfide higher. The polyarylene sulfide is dark brown in color so that the degree of whiteness is very low. As well, it is very brittle because of foaming due to thermal decomposition, an excessive degree of branching and cross-linking, and non-uniformity. The resin cannot be used singly, and a color matching is difficult due to its dark color. For these reasons, the use of the polyarylene sulfide is limited to a relatively narrow field.
The polyarylene sulfides of the linear high-molecular-weight type are superior in terms of the mechanical strength, and particularly properties associated with toughness, compared to those of the thermally cross-linked type, however, they are slow in crystallization as well as low in melt elasticity(melt tension). The polyarylene sulfides of this type are further poor in productivity because molding flash are likely to occur upon injection molding.
In order to solve the problems associated with the thermally cross-linked type and with the linear high-molecular-weight type in a complementary manner, it is necessary to attain an appropriate branching and cross-linking degree, as well as to optimize a molecular weight and a distribution of molecular weights.
It is known as a means for enlarging a molecular weight that the water content in the reaction mixture to be used for polymerization is adjusted by subjecting a hydrous alkali metal sulfide to dehydration prior to polymerization. However, it is to be noted that this procedure does not sufficiently enlarge the molecular weight of a resulting polymer. Thus, for example, U.S. Pat. No. 4,038,261 discloses a process in which a branching agent is added to enlarge the molecular weight of the resulting polymer. This process can produce a polyarylene sulfide such as polyphenylene sulfide having a sufficiently enlarged molecular weight with a relatively readiness, yet its melt flowability is reduced due to polymerization. It is thus to be considered to raise the melt flowability by elevating a molding temperature, however, an elevated molding temperature may cause a decomposition of the polymer so that a injection pressure upon injection molding should be elevated. As a result, molding flash are likely to occur on molded articles and items.
Japanese Patent Publication (laid-open) No. 223,753/1984 discloses a composition obtainable by blending a polyarylene sulfide of a linear high-molecular-weight type with a polyarylene sulfide of a thermally cross-linked type. This composition poses the problems that it is poor in thermal stability and low in a degree of whiteness.
Furthermore, Japanese Patent Publication (laid-open) No. 240,359/1987 discloses a composition consisting of a blend of a linear high-molecular-weight polyarylene sulfide with a linear low-molecular-weight polyarylene sulfide. This composition develops crystallization to a some extent, however, its melt elasticity is rather reduced, incurring the risk of mold flashing upon molding.
Demands have accordingly been made to provide a polyarylene sulfide that attains appropriate branching and cross-linking degrees, optimizes its molecular weight and a distribution of its molecular weights, and furthermore solves the problems encountered with the conventional thermally cross-linked and linear high-molecular-weight polyarylene sulfides.